I'm working on this English homework where I have to translate a text from my native language into English. There's one word that means "something that requires numerous attempts" or "something that requires retrying." I cannot say "trial and error" because the original text didn't really mention anything about errors. It is basically a sentence literally translated into something like " X requires retrying.". So my question is, is there a word to describe that? For example,
X is ___.
X requires ___ [something better than retrying].